Jet drive power requirement

Hello everyone, new here, first post. I'm considering trying to build a 1- off jet drive system and wonder if I could get some input on needed power / torque for my desired performance. The boat is roughly 14 feet, shallow draft, approximately 3-4 inches loaded. Combined boat,passenger,cargo, and motor weight maximum is 550 lbs. I would like the jet to be able to acheive a top speed of approx 20-25 mph at full load. I was planning on "frankensteining" together some old jet ski parts with a small engine. The only manufactured boat I'm aware of that is anything remotely close to what I'm thinking of is the Mokai; it runs a 7 hp Subaru that max's out at ~4500-4700rpm. But that boat will only run about 14-15 mph carrying a 250 lb man. I've been considering marrying a Honda scooter engine between 150-250cc's to the jet drive, they are water cooled and can turn almost twice the rpm. I'm looking for help determining what my hp/ torque requirements are to push ~550 lb boat in the 20-25 mph range. Thanks for any input, Crocogater.

 

Your engine needs to match the jet unit, you should find out the HP and rpm characteristics of the engine that was previously fitted to the jet unit, and not deviate too far from that.

 

Sensible, obvious answer. But here is my dilema: The boat has a maximum hp rating of 5, many people run 7’s with no problem. To covert to jet, i would need 9-10 hp engine to get 6-7 equivalent. Even to oldest jet skis were running 24-25 hp engines and as such that is what the jets were designed for. Does anyone know of a jet designed for 10-15 hp?Mokai has run 5 & 7 hp engines with their designed jet; but even if that was feasible i understand they dont sell to “outsiders”. How well would a jet perform running an engine only 1/2 the hp it was designed for? I dont need the performance of the jet ski, but would appreciate an educated estimate at what i may get to decide if the project is worth proceeding

 

The jet pump characteristics follows mathematical rules called the affinity laws: the Power varies with rpm^3. So, if you want to use a specific jet/impeller combination for 50% of its design power, you have to operate it at (0,5)^(1/3) ~79% of the design rpms. At this shaft speed the pump efficiency is roughly equal to the design efficiency, provided the boat can reach ~79% of its design speed with the reduced power output.

 

Berkekey used to manufacture a series of little jets; they can still be found used. I have one of the 5j5 models that I've been saving for some future project; this link shows that unit's old specifications with ratings down to as low as 3 HP.

In fact, there is a used 6J on eBay right now.

Berk 5J5, 6JA, 6JB https://www.tapatalk.com/groups/norcalhotboats/berk-5j5-6ja-6jb-t47.html